Taxes are a main source of a state's financial income, and sales taxes provide an important contribution to the overall tax revenue. Sales taxes are imposed according to the total sales declared by merchants. Therefore, the procedures used by merchants to declare taxes are important. Some merchants conceal parts of their trade in order to evade sales taxes, especially when payments are in cash, since there is no traceable bank record of such cash payments, and tax offices will therefore not be able to discover them. Such unreported cash transactions thereby reduce the state's tax income. Therefore, there is a problem of how to prevent merchants from concealing taxable trades.